1. Field of the Invention
This invention relates to a temperature detection circuit used in a thermal shielding circuit, and more particularly to a temperature detection circuit provided together with an integrated circuit having a large power consumption on the same chip, for protecting the circuit from being damaged by heat generated therefrom.
2. Description of the Related Art
Generally, in a case where a circuit such as an acoustic power amplifier and a motor driver circuit having a relatively large power consumption is integrated, the amount of heat generated therefrom is large and the whole circuit is heated, and therefore, much attention should be paid to heat generation than in a case wherein discrete semiconductor elements are used. Therefore, the resistance of an outer casing of the integrated circuit is lowered and a heat radiating plate is used, and in addition, a thermal shielding circuit using a temperature detection circuit is provided to interrupt the circuit operation and protect the integrated circuit in an abnormal high temperature condition.
In FIG. 1, a conventional thermal shielding circuit widely used in an acoustic power amplifier circuit formed in an integrated circuit form is shown. The thermal shielding circuit is constructed to include resistors R.sub.1, R.sub.2 and R.sub.3, Zener diode Z.sub.1 and NPN transistors Q.sub.1 and Q.sub.2. The resistor R.sub.1 and Zener diode Z.sub.1 are series-connected between a power source Vcc and a ground terminal GND. The base of the transistor Q.sub.1 is connected to a connection node between the resistor R.sub.1 and the Zener diode Z.sub.1 and the collector thereof is connected to the power source Vcc. The resistors R.sub.2 and R.sub.3 are series-connected between the emitter of the transistor Q.sub.1 and the ground terminal GND. The base of the switching transistor Q.sub.2 is connected to a connection node between the resistors R.sub.2 and R.sub.3 and the emitter thereof is connected to the ground terminal GND. A shield control signal is output from the collector of the transistor Q.sub.2.
The resistor R.sub.1 is provided to determine the magnitude of a bias current supplied to the Zener diode Z.sub.1. The resistors R.sub.2 and R.sub.3 are used to set the detection temperature, are formed of the same type of resistors and are designed by taking the pairing property such as the temperature characteristic into consideration so as to set the ratio of the resistance thereof to a constant value.
The operation of the thermal shield circuit shown in FIG. 1 is effected as is well known in the art so as to detect the temperature by using the temperature characteristics (having a negative temperature characteristic) of base-emitter voltages V.sub.F1 and V.sub.F2 of the bipolar transistors Q.sub.1 and Q.sub.2 and the Zener voltage Vz (having a positive temperature characteristic) of the Zener diode Z.sub.1, output a shield control signal when an abnormal high temperature condition is detected, and then interrupt operation of a circuit connected to the succeeding stage. That is, in the normal temperature condition, the base potential of the transistor Q.sub.2 which is determined by the Zener voltage Vz, the base-emitter voltage V.sub.F1 of the transistor Q.sub.1 and the ratio of the resistances of the resistors R.sub.2 and R.sub.3 is set to be lower than the base-emitter voltage V.sub.F2 of the transistor Q.sub.2. As a result, the transistor Q.sub.2 is kept in the OFF state at the normal temperature and the circuit connected to the succeeding stage effects the normal operation.
In contrast, when the temperature of the integrated circuit chip rises, the ratio of the resistances of the resistors R.sub.2 and R.sub.3 can be kept constant even if the resistances thereof are changed by the temperature rise because the resistors R.sub.2 and R.sub.3 are of the same type of resistor, but at this time, the Zener voltage Vz becomes high and the base-emitter voltage V.sub.F1 of the transistor Q.sub.1 becomes low. As a result, the emitter potential of the transistor Q.sub.1 rises and at the same time the base potential of the transistor Q.sub.2 also rises. The base-emitter voltage V.sub.F2 necessary for turning on the transistor Q.sub.2 is lowered so that the transistor Q.sub.2 may be set in the ON state (or in the saturation state) when the preset temperature is exceeded. The turn-on of the transistor Q.sub.2 causes the operation of the circuit connected to the succeeding stage to be interrupted.
The conventional thermal shielding circuit described above supplies a shield control signal to interrupt the operation of the power amplifier circuit when the preset temperature is exceeded, interrupts the protecting operation for the power amplifier circuit when the temperature becomes lower than the preset temperature, and thus the operation of the power amplifier circuit is instantaneously recovered. Therefore, when the protecting operation and recovering operation of the power amplifier circuit are repeatedly effected, an oscillation state is set up with the temperature kept at or near the preset temperature. When the oscillation state is thus set up, a bad influence may be sometimes given to peripheral circuits, and it is not preferable to keep the power amplifier circuit in the protecting state with the temperature kept at the high temperature.
Further, if variation occurs in the Zener voltage Vz of the Zener diode Z.sub.1 in the manufacturing process, the base potential of the transistor Q.sub.2 varies, causing the preset potential for the protecting operation to be varied.
The conventional thermal shielding circuit described above has a defect that the integrated circuit chip is set into the oscillation state when the protecting operation and recovering or restarting operation of the circuit which is to be protected and which generates heat are repeatedly effected while the temperature is kept at the high temperature near the shielding temperature. Further, if variation occurs in the Zener voltage of the Zener diode, the preset potential for the protecting operation may be varied.